Optical fibre comprising a bragg grating and tunability means

ABSTRACT

The invention concerns an optical device comprising an optical fiber ( 10 ) including at least an integrated component, a support ( 40 ) including two fixing zones ( 30, 32 ) whereon the optical fiber ( 10 ) is fixed respectively at the two zones ( 12, 14 ) located on either side of the component. The invention is characterized in that the support ( 40 ) comprises, between the two fixing zones ( 30, 32 ), at least a linking beam ( 22, 23, 24 ) capable of bending and the device further comprises means ( 50 ) for imposing a controlled bending to the beam ( 22,23,24 ), adapted to adjust the distance between the two fixing zones ( 30, 32 ).

[0001] The present invention relates to the field of optical fibers.

[0002] Even more precisely, the present invention relates to the fieldof optical fibers which include an integrated component.

[0003] The present invention applies in particular to fiber-opticdevices which include an integrated Bragg grating. Within this context,the invention aims to provide a device for temperature stabilizationand/or Bragg wavelength adjustment of the gratings photowrittten in theoptical fibers.

[0004] A person skilled in the art knows that it is possible toconsiderably increase the traffic of optical-fiber networks by thetechnique of “wavelength multiplexing”. Each series of data to betransported is transmitted over one specific optical frequency,multiplying the capacity of the fiber by the number of wavelengths used.

[0005] The number of multiplexible wavelengths in a given frequency bandcannot, however, be increased ad infinitum. It depends on the levels ofdrift and on the degrees of precision of the devices capable ofinserting a wavelength into the fiber or extracting it therefrom,without disturbing the flux of the other ones.

[0006] Among devices used, some rely on Bragg gratings written into thecore of the fiber.

[0007] Bragg gratings are periodic optical index structures which havethe particular feature of reflecting light of a well-defined wavelength,called the Bragg wavelength of the grating. Systems based on Bragggratings have already been of great service. However, it turns out thatthis reflected wavelength, which depends on the periodicity of thegrating and on the parameters of propagation in the fiber, is:

[0008] an increasing function of temperature and

[0009] an increasing function of the tension applied to the fiber.

[0010] Means for limiting the temperature drift of devices using Bragggratings are known. The means most in use at the present time are knownas tabletop or semi-tabletop arrangements. A description of illustrativeexamples of these means will be found in documents [1] and [2].

[0011] The appended FIGS. 1 and 2 show tabletop and semi-tabletopstructures according to the prior art, respectively.

[0012] Tabletop or semi-tabletop arrangements consist of a beam 20 madeof a material having a low expansion coefficient—invar, ceramic,etc.—and of one or two blocks 30, 32 made of a material having a highexpansion coefficient, for example aluminum. The fibers 10 which includea Bragg grating are mounted so as to be held taught between the twoblocks 30, 32 in the case of a tabletop or between the block 30 and theopposite end of the beam 20 in the case of a semi-tabletop, depending onthe type of arrangement. The points for fixing the fiber 10 are labeled12 and 14.

[0013] The distance between these two points 12, 14 decreases withincreasing temperature and the tension applied to the fiber decreasesand tends to reduce the Bragg wavelength, whereas, since temperatureinduces the opposite effect, an equilibrium is established between thetwo phenomena.

[0014] For this type of device to be stable over time requires the Bragggrating to be definitively held in place in this arrangement by adhesivebonding or welding. Any error between the Bragg wavelength obtained andthe desired wavelength then becomes uncorrectable.

[0015] Moreover a person skilled in the art knows that the opticalsignal propagating in a fiber is attenuated over the course of its path.After a certain distance, it then becomes necessary to reamplify it.Many of the amplifiers used are fiber amplifiers which have the drawbackof amplifying the light passing through them to a degree which varieswith wavelength. To equalize the output levels of the variouswavelengths, a gain-equalizing filter is added to these devices.

[0016] Gain-equalizing filters based on Bragg gratings consist of asuccession of gratings with slanted lines. Each of these gratingsextracts from the fiber a small portion of the light in the wavelengthband corresponding to it, this amounting to producing a certainabsorption in this band. The distribution of these various absorptionsallows the output level of the light to be evened out, independently ofits wavelength.

[0017] There is one tension value to be applied to the gain-equalizingfilter in order to obtain the optimum result or, formulated another way,there is a value of the overall wavelength shift to be applied to thefilter in order to have the flattest possible response.

[0018] In general, certain optical functions may require the Bragggratings to be adjusted as desired; these are thus tunable gratings.

[0019] The objective of the invention is to provide a means of adjustingthe response of fiber-optic-based devices which include an integratedcomponent.

[0020] In particular, the objective of the present invention is toprovide a means of adjusting the Bragg wavelength of tabletop orsemi-tabletop arrangements.

[0021] The aforementioned objectives are achieved within the context ofthe present invention by a fiber-optic device comprising:

[0022] an optical fiber which includes at least one integratedcomponent,

[0023] a support having two fixing regions to which the optical fiber isfixed at the two regions lying respectively on each side of thecomponent,

[0024] characterized in that:

[0025] the support comprises, between the two fixing regions, at leastone linking beam capable of bending, and in that

[0026] the device furthermore comprises means which can impose acontrolled bending on the beam, suitable for adjusting the distancebetween the two fixing regions.

[0027] Further features, objectives and advantages of the presentinvention will become apparent on reading the detailed description whichfollows and in conjunction with the appended drawings, given by way ofnonlimiting examples and in which:

[0028]FIGS. 1 and 2, described above, show schematically twoarrangements, in tabletop and semi-tabletop form, according to the priorart;

[0029]FIG. 3 shows a device according to the present invention, at rest;

[0030]FIG. 4 shows schematically the same device according to thepresent invention, during an adjustment phase; and

[0031] FIGS. 5 to 7 show three variants according to the presentinvention,

[0032]FIG. 7 showing a view in a plane orthogonal to FIGS. 3 to 6.

[0033] FIGS. 3 to 7 according to the present invention show a devicecomprising an optical fiber 10 fixed at 12, 14 to a support 40.

[0034] The support 40 comprises a beam 20 made of a material having alow expansion coefficient—invar, ceramic or equivalent, etc.—and,depending on whether it is a tabletop or a semi-tabletop arrangement,one or two blocks 30, 32 made of a material having a high expansioncoefficient, for example aluminum.

[0035] The fibers 10 which include a Bragg grating are mounted so as tobe held taught between the two blocks 30, 32 in the case of a tabletopor between the block 30 and the opposite end of the beam 20 in the caseof a semi-tabletop.

[0036] The materials making up the support 40 and their dimensions aretailored so that the effect of temperature on the Bragg grating and onthe reflected wavelength is compensated for by the variation in tensionin the fiber (the variation in distance between the two fixing points12, 14, which distance decreases with increasing temperature, and theresulting variation in the tension applied to the fiber decrease andtend to reduce the Bragg wavelength, whereas temperature intrinsicallyinduces the opposite effect on the Bragg grating).

[0037] These arrangements, well known in themselves to those skilled inthe art, will not be described in detail below.

[0038] However, as indicated above, according to the present invention:

[0039] the support 40 comprises, between the two fixing regions 12 and14, at least one linking beam 20 capable of bending; and

[0040] the device furthermore comprises means, shown schematically bythe reference 50 in FIG. 4, which can impose a controlled bending on thebeam 20, suitable for adjusting the distance between the two fixingregions 12, 14.

[0041] When a bending force is applied to the beam 20, the fixed points12, 14 of the Bragg grating formed in the fiber 10 are brought closertogether. The relaxation of the tension which results therefrom reducesthe wavelength to a defined value.

[0042] According to the embodiment in accordance with the presentinvention illustrated in FIG. 3, that region of the support 40 which islocated between the two fixing regions 12 and 14 consists of a beam 20recessed at its center. Thus, the beam is formed from two parallelsidewalls or bars 22, 23.

[0043] More precisely, according to the embodiment illustrated in FIG.3, the central recess 21 delimiting the bars 22, 23 is oblong anddefines bars 22, 23 having at least, approximately the same crosssection over their entire length. However, the invention is not limitedto this recess geometry 21, nor to bars 22, 23 of constant crosssection. Thus, it may be envisioned to have recesses defining acontrolled variation in the cross section of the bars 22, 23 in order tocontrol the bending region of the latter.

[0044] However, it should be noted that it is preferable within thecontext of the present invention, for the beam 20 providing the linkbetween the two fixing regions 12, 14 to be symmetrical with respect toa longitudinal plane passing through the axis of the fiber 10. Thissymmetry, whatever the bending applied to the beam 20, prevents harmfulmechanical stresses being applied to the fiber 10.

[0045] The means 50 capable of controlling the bending of the beam 20,and more precisely of bringing the sidewalls 22, 23 of the said beam 20closer together, may be the subject of many embodiments.

[0046] According to a first embodiment illustrated in FIG. 5, thesemeans 50 for controlling the bending by bringing the sidewalls 22, 23 ofthe recessed beam 20 closer together, comprise a screw 52 transverse tothe beam.

[0047] Such a screw 52 may comprise two portions of reverse pitch whichrespectively engage with one of the sidewalls 22 and 23, or else asingle threaded portion which engages with one of the sidewalls 22 or23, a head or stop on the screw resting against the other sidewall 23 or22.

[0048] According to yet another embodiment, the screw 52 may be designedto control the bending of the beam 20 by moving the sidewalls 22, 23further apart, and not closer together.

[0049]FIG. 6 illustrates another embodiment according to the presentinvention, whereby the beam 20, delimited in the two symmetricalsidewalls 22, 23 by a central recess 21, has a conical overall envelope.In other words, the beam 20 has a cross section which increases from thefixing region 12 toward the fixing region 14. In this case, the meansfor adjusting the bending is formed by a ring 52, the internal crosssection of which is complementary to the external cross section of thebeam 20 in its region of smallest cross section and which ring can bemoved along the beam 20. A person skilled in the art will understandthat the bending of the beam is a minimum when the ring 55 is close tothe first fixing region 12 and, in contrast, the bending of the beam 20is a maximum when the ring 55 is close to the fixing region 14.

[0050] According to the embodiments described above, the beam 20 iscomposed of two symmetrical bars 22, 23. As a variant however, devicesaccording to the present invention may be produced with the aid of asingle linking bar.

[0051] Moreover, according to the embodiments described above, the beam20 is deformed in bending by deformation of the beam or of the bars 22,23 in a direction perpendicular to the plane of symmetry of the fixingregions 12, 14 (which plane of symmetry is perpendicular to the plane ofFIGS. 3 to 6).

[0052] However, as a variant, it may be envisioned to control thebending of the beam 20 by deformation in a direction parallel to theplane of symmetry of the fixing regions 12, 14. An embodiment for thispurpose is illustrated in FIG. 7.

[0053] This FIG. 7 illustrates a variant which includes a screw 56 whoseaxis lies in the aforementioned plane of symmetry, the said screwengaging with a bar 24 located between the two fixing regions 12, 14,while its head rests on a rigid support or on a second symmetrical bar,in order to control at least the bending of the bar 24.

[0054] Of course, the present invention is not limited to the particularembodiment that has just been described, but encompasses all variantsfalling within its spirit.

[0055] It should be noted, for example, that within the context of thepresent invention the various means for adjusting the bending of thesupport 40 may be controlled manually or by a motor drive.

BIBLIOGRAPHY

[0056] [1]V. FLEURY “Stabilisation thermique des réseaux de Braggphotoinscrits sur fibre optique [Thermostabilization of photowrittenBragg gratings in an optical fiber]”, DESS INGENERIE LASER TrainingCourse Report at the Université des Sciences et Technologies de Lille[Lille University of Sciences and Technologies].

[0057] [2]J. Rioublanc et al. “Optimisation d'un système destabilisation passive de la dérive en température de la longueur d'onded'accord des réseaux de Bragg [Optimization of a system for the passivestabilization of the temperature drift of the tuning wavelength of Bragggratings]”, JNOG96, Paper No. 85.

1. A fiber-optic device comprising: an optical fiber (10) which includesat least one integrated component, a support (40) having two fixingregions (30, 32) to which the optical fiber (10) is fixed at the tworegions (12, 14) lying respectively on each side of the component,characterized in that: the support (40) comprises, between the twofixing regions (30, 32), at least one linking beam (22, 23; 24) capableof bending, and in that the device furthermore comprises means (50)which can impose a controlled bending on the beam (22, 23; 24), suitablefor adjusting the distance between the two fixing regions (30, 32). 2.The device as claimed in claim 1, characterized in that the fiber (10)includes an integrated Bragg grating.
 3. The device as claimed in eitherof claims 1 and 2, characterized in that the support (40) comprises abeam (20) made of a material having a low expansion coefficient and oneor two blocks (30, 32) made of a material having a high expansioncoefficient forming the fixing regions.
 4. The device as claimed inclaim 3, characterized in that the fiber (10) is held taught between twoblocks (30, 32).
 5. The device as claimed in claim 3, characterized inthat the fiber (10) is held taught between a block (30) and the oppositeend of the beam (20).
 6. The device as claimed in one of claims 1 to 5,characterized in that the beam capable of bending consists of a beam(20) recessed at its center.
 7. The device as claimed in one of claims 1to 6, characterized in that the beam capable of bending is formed fromtwo parallel sidewalls or bars (22, 23).
 8. The device as claimed inclaim 7, characterized in that the bars (22, 23) have at leastapproximately the same cross section over their entire length.
 9. Thedevice as claimed in claim 7, characterized in that the bars (22, 23)have a cross section which varies over their length.
 10. The device asclaimed in one of claims 1 to 5, characterized in that the beam capableof bending consists of a single beam (20).
 11. The device as claimed inone of claims 1 to 10, characterized in that the linking beam (20) issymmetrical with respect to a longitudinal plane passing through theaxis of the fiber (10).
 12. The device as claimed in one of claims 1 to11, characterized in that the bending control means (50) are designed todeform the linking beam (20) in a direction perpendicular to the planeof symmetry of the fixing regions (12, 14).
 13. The device as claimed inone of claims 1 to 11, characterized in that the bending control means(50) are designed to deform the linking beam (20) in a directionparallel to the plane of symmetry of the fixing regions (12, 14). 14.The device as claimed in one of claims 1 to 13, characterized in thatthe bending control means (50) comprise a screw (52, 55) transverse tothe beam.
 15. The device as claimed in claim 14, characterized in thatthe screw (52) engages with two sidewalls (22, 23) of the linking beam(20).
 16. The device as claimed in claim 14, characterized in that thescrew (55) engages with a bar (24) and a rigid support.
 17. The deviceas claimed in one of claims 1 to 16, characterized in that the linkingbeam (20) is delimited in two sidewalls (22, 23) by a central recess(21) and has a conical overall envelope, and the means for adjusting thebending is formed by a ring (55), the internal cross section of which iscomplementary to the external cross section of the beam (20) in itsregion of smallest cross section, which ring can be moved along the beam(20).
 18. The device as claimed in one of claims 1 to 17, characterizedin that the bending control means are designed to operate by bringingthe two sidewalls (22, 23) of the linking beam (20) closer together in acontrolled manner.
 19. The device as claimed in one of claims 1 to 17,characterized in that the bending control means are designed to operateby moving the two sidewalls (22, 23) of the linking beam (20) furtherapart in a controlled manner.
 20. The device as claimed in one of claims1 to 19, characterized in that the bending control means (50) aredesigned for manual operation.
 21. The device as claimed in one ofclaims 1 to 19, characterized in that the bending control means (50) arelinked to a motor drive.